This invention relates generally to an electro-optical device, and more particularly to an electro-optical device which includes a field effect type liquid crystal display cell and an anisotropic material disposed between a pair of polarizers.
Conventional electro-optical devices which include super twisted nematic (STN) liquid crystal cells exhibit other than substantially flat transmittance characteristics with respect to wavelength. Such non-flat transmittance characteristics in a positive type display cause white light to be displayed as green, yellowish green, yellow or reddish yellow when the device is turned off and blue or dark blue when the device is turned on. Similarly, white light passing through a negative type display device will be displayed as blue or dark blue when turned off and yellow when the device is turned on. In other words, conventional electro-optical devices operating in a STN mode are unable to display monochromatic black and white from white incident light.
To prevent such coloring, use of an achromatic, optically anisotropic substance or other type of compensating cell can be used. More particularly, light rays divided by double refraction occurring within the STN liquid crystal cell cause linealy polarized light to become elliptically polarized light. By passing the elliptically polarized light through the optically anisotropic substance the colored light is converted once again to white light. In other words, the optically anisotropic substance serves to compensate for coloring caused by the double refraction of light rays passing through the STN liquid crystal cell.
As shown in FIG. 3, an electro-optical device 50 employing a conventional STN mode of operation includes an analyzer or polarizer 1, a compensating cell 2, a STN liquid crystal cell 3 and a polarizer 4. An incident light 25 initially enters device 50 through polarizer 4 and leaves device 50 through analyzer 1. Incident light 25 is neither polarized nor homogeneous and includes a plurality of light rays 251 which are substantially perpendicular to the direction of propagation. Analyzer 1 has an axis of polarization 19 which is substantially perpendicular to an axis of polarization 18 of polarizer 4. As incident light 25 passes through polarizer 4, incident light 25 becomes a linearly polarized light 26 which includes different wavelengths such as, but not limited to, light of a blue wavelength 261, a green wavelength 262 and a red wavelength 263. All wavelengths of linearly polarized light 26 propagate in substantially the same direction as the direction of polarizing axis 18.
As linearly polarized light 26 passes through STN liquid crystal cell 3, linearly polarized light 26 is transformed to an elliptically polarized light 27. More particularly, a plurality of light rays 261, 262 and 263 of linearly polarized light 26 are transformed into a plurality of elliptically polarized light rays 271, 272 and 273, respectively, based on the birefringence of STN cell 3. Each elliptically polarized light ray differs in its state (e.g. direction of its axes) according to its wavelength (i.e. color). Assuming for a moment that device 50 does not include compensating cell 2, elliptically polarized light 27 then would be fed directly to analyzer 1. The amount of light transmitted through analyzer 1 depends on how much of the wavelength (i.e. color) of each light ray within elliptically polarized light 27 is in the same direction as polarizing axis 19. Since each of the wavelengths differ in state, a light 29 emitted from analyzer 1 would be colored, that is, other than white even though incident light 25 enters device 50 as white light. Such coloring can be avoided by passing polarized light 27 through compensating cell 2. The elliptically polarized light rays of light 27 are converted back to a linearly polarized light 28. Linearly polarized light 28 includes, but is not limited to, light of wavelengths 281, 282 and 283 which correspond to light of wavelengths 271, 272 and 273, respectively. When the direction of linearly polarized light 28 is substantially perpendicular to the direction of polarizing axis 19 of analyzer 1 (i.e. when no voltage is applied to STN liquid crystal cell 3), very little, if any portion of linearly polarized light 28 passes through analyzer 1. Therefore, black characters or digits can be displayed by device 50. By applying a voltage across STN liquid crystal cell 3, the direction of linearly polarized light 28 is substantially parallel to the direction of polarizing axis 19 of analyzer 1. Accordingly, white characters or digits can be displayed by device 50.
Electro-optical device 50 requires that compensating cell 2 must be made from a liquid crystal composition which is identical to the liquid crystal composition of STN liquid crystal cell 3 with respect to material, layer thickness (i.e. cell gap) and twist angle to provide a colorless, substantially perfect black and white (i.e. monochromatic) display. The liquid crystal material of compensating cell 2 also must be twisted in a direction opposite to the direction in which the liquid crystal material of STN liquid crystal cell 3 is twisted.
The optical path travelled during double refraction is defined as the product of an anisotropy (.DELTA. n) of the refractive index of a liquid crystal material and a layer of thickness d (i.e. .DELTA. nd). When compensating cell 2 and STN liquid crystal cell 3 are made from the same liquid crystal material, each has the same refractive index anisotropy .DELTA. n. Since layer thickness d (i.e. cell gap) of cells 2 and 3 are the same, the product of .DELTA. nd for each cell is the same.
Expensive additives or other agents are typically added to the liquid crystal composition of STN liquid crystal cell 3 and compensating cell 2 to provide an acceptable level of responsiveness and suitable temperature characteristics. These additives or other agents increase the material cost and add to the manufacturing process complexity of device 50. The time required to fabricate cells 3 and 2 also increases. STN liquid crystal cell 3 is also typically made from an expensive liquid crystal material. Conventional electro-optical devices use the same expensive liquid crystal material for compensating cell 2 resulting in a further increase in material costs associated with device 50.
Until now it also has been considered that layer thickness d of compensating cell 2 must be equal to layer thickness d of STN liquid crystal cell 3. To avoid uneveness in the thickness of STN liquid crystal cell 3, extremely thin cells are avoided. Consequently, layer thickness d of compensating cell 2 cannot be extremely small.
Accordingly, it is desirable to provide an electro-optical device using a STN liquid crystal display cell and compensating cell in which the twist angles, anisotropy .DELTA. n, layer thickness d and a refractive index dispersion .alpha. (to be discussed below) of each cell are not necessarily the same. More particularly, one or more of these parameters can differ in value in each cell while providing a suitable display with high contrast (i.e. a monochromatic black and white display). It is also desirable to provide an electro-optical device which includes a compensating cell made from a variety of different liquid crystalline materials in which the manufacturing steps for adjusting the cell gaps is simplified.